The validity of a new model of the mechanism by which CO2 is transported in the blood will be empirically tested. The new model suggests that the movement of chloride ions during gas exchange, the "chloride shift", is caused by changes of the PO2-dependent affinity of hemoglobin for chloride that has been demonstrated by NMR studies, (but also by classical chemical techniques; Guesonon et al, Resp. Physiol. 38:115-129,1979). Thus, there are two pools of chloride in the cell, free and bound. As O2 leaves the red cell, the concentration of the pool of free chloride decreases as it binds to hemoglobin, causing chloride ion to diffuse into the red cell from the plasma; the opposite occurs during oxygenation. The formation of bicarbonate is now known to occur directly in the plasma, catalyzed by carbonic anhydrase bound to the capillary endothelium. Thus, there is no need to require passage of bicarbonate ions through the red cell. The plasma can only hold the known amounts of bicarbonate if the complimentary anion, e.g., chloride is removed. Similarly, if the chloride that enters the red cell were not bound by hemoglobin from the free chloride pool, the measured amounts of bicarbonate that are formed in the red cell chemically could not occur. To test the model it must be demonstrated: (1) that there are separate pools of chloride in the red cell; (2) that the size of the pools changes appropriately as a function of PO2, and (3) that the effects are physiologically meaningful. Concentrations of chloride in the free and bound pools will be measured by NMR as PO2 is varied. Free and bound chloride may also be separated by centrifugation of isolated RBC cytosols that have been equilibrated with differing PO2 in tubes with filtration membranes that exclude hemoglobin. Chloride content can be subsequently analyzed in the two portions. Results will be compared with standard measurements. Demonstration of the validity of the new model will correct the current model of CO2 transport that has been accepted and taught for more than sixty years.